... all of this trouble happened, the Planetary Society blog had a nice detailed writeup [planetary.org] a while back. The "short" of it? Akatsuki has a new type of primary thruster based on ceramics to withstand the heat rather than exotic materials like dicilicide-lined niobium as are normally used on these sorts of small hypergolic thrusters; they wanted to prove the new technology. You generally run thrusters a bit fuel-rich and inject it in such a manner as to try limit combustion near the chamber and nozzle walls to keep the temperature down. Well, the pressurant valve to the fuel tank didn't open all the way (they think it corroded) but the oxidizer pressure valve opened all the way. So the burn kept getting more and more oxidizer rich, meaning hotter chamber and nozzle walls way past the design limits, until they cracked and the nozzle simply flew off.

The only reason they were able to salvage this was because another unusual choice they did: to save mass, they implemented a more complicated hydrazine (fuel) feed system, allowing them to use the same hydrazine suppy for the main engine as for the small monopropellant RCS thrusters (tiny, low-efficiency maneuvering thrusters). Because they did this, they were able to take the fuel that was planned for the main engine and route it instead to the RCS engines. While they're less efficient and much lower thrust, they had enough excess fuel to pull off the maneuver (after first making the craft lighter by dumping the now-unneeded oxidizer, of course!)

Hoping I have this stuff right, please correct me if I'm wrong...If hydrazine is a monopropellant (energetically decomposes with a catalyst), and they used it for both RCS and their main thruster, then why was there oxidizer at all? Is it an efficiency thing where it burns better with oxygen, or am I missing something?

(Incidentally, oxygen would not be the oxidizer of choice for a deep-space mission, since it will boil off -- dinitrogen tetroxide is the usual choice to go with hydrazine. Liquid at room temperature, with ignition on contact. Also horribly toxic.)

Nice summary! I would note that the ability to interconnect the bipropellant hydrazine supply to the monopropellant engines is not an "unusual choice", it's common bordering on ubiquitous. Pretty much in order to provide this option as a backup.And it has been used in exactly this way (for more-or-less the same reason) in the very recent past.

I can think of a couple examples of spacecraft where it's been used that way (the Planetary Society's writeup mentions MESSENGER and MRO), but not many. Most of the designs I've seen use separate systems, for simplicity and supposedly extra reliability (the irony here being that the lack of separation on the systems is what saved Akatsuki). Cassini, for example has fully independent systems - like Akatsuki its main engine is hydrazine/N2O4 and its RCS is monoprop hydrazine, but the RCS has its own separate tank.

The grand old lady was known in Homer's Illiad as 'white armed Hera', the wife of Zeus.

Yes and Zeus and his sister Hera had a child which made Zeus jealous thinking that it would become a more powerful god than he. So he ate it. SOMEHOW I don't incest. child murder and cannibalism is how the Japanese managed to pull off the engineering marvel of using slow thrust over 5 years to salvage their mission to Venus though. But it might be how the US manages to send a manned mission to Mars if Donald Trump gets elected. Come to think of it he and his supporters are welcome to claim Mars provided th